Definition
A binary numbering system in which only one bit changes between any two consecutive values. In aviation, Gray Code is the format used by Mode C transponders to transmit pressure altitude to ATC radar in 100-foot increments.
Plain English
A special way of counting in 1s and 0s where only one digit changes at a time when the number goes up or down by one. The transponder uses this method to send altitude information to air traffic control without errors.
Context Anchor
Seen in avionics, especially older altitude encoder and transponder systems that send pressure altitude as a set of electrical on/off signals.
Derivation
Named after Frank Gray, a Bell Labs researcher who patented the code in 1953. It is sometimes called 'reflected binary code' because of the way the bit pattern mirrors itself as the numbers grow.
Why Pilots Care
Prevents momentary false altitude readings during bit transitions in the encoder output.
Analogy
Think of a row of light switches used to show a number. In ordinary numbering, several switches might flip at the same time, and someone looking during the flip could see the wrong number. Gray Code arranges the numbers so only one switch flips at a time.
Intuition Check
Gray Code does not mean a color-coded wire system. It is a numbering method named after Frank Gray, designed so only one electrical signal changes at each step.
Example Sentence 1
The encoding altimeter converts the aircraft's pressure altitude into Gray Code and sends it to the transponder for transmission to ATC.
Example Sentence 2
A Gray Code transition changes only one bit, avoiding the altitude jumps that would occur with ordinary binary counting.